With reference to FIG. 1, for x-ray analysis in an electron microscope (EM) 100, an x-ray spectrum is measured by sensing and measuring the energies of individual x-ray photons emitted by a specimen 101 when it is hit by a focussed electron beam 102. (Note that in this document, the convention is that the electron beam travels vertically downwards towards the specimen and this is the context for words such as “below” and “above”. In practice the electron beam can be oriented in any direction, including vertically upwards.) Each x-ray photon is an energetic particle and the energy is typically converted into charge using a solid state detector 105. The charge is measured so that a count can be recorded and the histogram of recorded measurements represents the digitised x-ray energy spectrum. Peaks characteristic of particular chemical elements can be identified in the x-ray energy spectrum and the intensity of those peaks used as the basis for determining elemental content of the material which is directly under the electron beam 102. The volume of material that is characterised is determined by the diameter of the focussed electron beam and the range of electrons as they scatter within the specimen 101. If the incident electron beam energy is 5 keV, the range of electron scattering is of the order of 100 nm for a bulk steel specimen. At higher incident electron energies, the range in a bulk specimen is greater but if the specimen is thinned so that the electron beam passes through the specimen, the sideways scattering can be reduced. Thus it is usually possible to find a set of conditions where x-ray analysis can be achieved on very small volumes of material provided the size of the focussed beam on the specimen is sufficiently small.
The x-ray detector 105, the final polepiece of the electron microscope 104 and the specimen 101 are all within the same vacuum chamber. The vacuum is primarily needed so that the electrons can be accelerated to several keV energy and focussed to a narrow beam without scattering on gas molecules. However, some specimens such as liquids and hydrated biological material will not be stable under vacuum conditions and cannot be analysed. Furthermore, specimens that are electrically insulating may accumulate charge and the negative voltage can eventually repel the incident beam so that analysis is not possible.
One approach to analysing “wet” samples or insulating samples is the “environmental scanning electron microscope” which uses a small aperture to isolate the “good” vacuum required for producing a focussed electron beam, from the specimen chamber which has gas at a fraction of atmospheric pressure. Since gas flows through the pressure limiting aperture, the microscope has to continually pump to maintain the level of vacuum in the electron column. If the specimen chamber pressure is high, the aperture diameter has to be reduced to allow the pumping to keep up with the leakage. For example, if a 1 mm aperture can just sustain a pressure of 5 Torr which is enough to keep water in its liquid phase, to work at 10 Torr may require a 0.5 mm aperture. However, to work at full atmospheric pressure (760 Torr) would require an aperture that severely limits the range of deflection for the electron beam. Nevertheless, operation at pressures even below 5 Torr is useful because ionised gas atoms help neutralise surface charge that builds up on insulating specimens which can therefore be analysed without having to coat the specimen with a conductive material.
U.S. Pat. No. 6,452,177 describes an alternative approach for analysing specimens in their natural atmosphere. An evacuated electron column generates, accelerates, and focuses electrons and is isolated from the ambient atmosphere by a thin, electron transparent membrane. After passing through the membrane, the electrons impinge on the sample in atmosphere, to generate characteristic x-rays. The x-rays are detected and analysed with a solid-state x-ray detector mounted near the sample and in the ambient atmosphere of the sample. When the electron beam emerges from the membrane it is scattered by gas molecules in the ambient atmosphere. With a 30 keV beam, the combined scattering in the membrane and atmospheric gas broadens the finely focussed beam into a broad spot 1 mm in diameter at 2 cm working distance from the specimen. Furthermore, even if the x-ray detector is fitted with a thin polymer window, x-rays are still absorbed in the gaseous atmosphere. For example, if the detector is only 3 mm away from the specimen, low energy x-rays are strongly absorbed by air at atmospheric pressure (1 Bar) as seen in FIG. 2.
The use of an electron-transparent membrane overcomes some of the problems associated with differential pumping apertures. Sealing the vacuum with a membrane avoids the need for continuous pumping. The membrane diameter can also be larger than that of the small pumping aperture required to sustain a differential of one atmosphere of pressure. Provided the membrane is thin and the electron beam energy is high enough, there is not much sideways scattering within the membrane. Furthermore, if the specimen is placed very near to the membrane, scattering off gas molecules is minimal so the incident electron beam remains focussed to a small spot.
In a microscope with differential pumping it is also advantageous to reduce scattering off gas molecules by moving the specimen very close to the pumping aperture. The pumping aperture or membrane will be close to the bottom face of the polepiece (104, FIG. 1) of the final lens. However, when the specimen (101) is placed very near to the polepiece (104) it is difficult for the detector (105) to get line of sight of the beam spot and consequently x-ray detection is inefficient.
Therefore, it is desirable to have an efficient method of detecting x-rays in a microscope that has been designed to accommodate specimens at atmospheric pressure or at sufficiently high pressure to work with liquid or non-conductive specimens.